The present invention relates to a closing unit according to the preamble of claim 1.
Standard injection moulding machines comprise closing units in which the clamping plates are guided by a plurality of bars. However, the use of closing units without bars is becoming more common today for certain applications, in particular in the production of miniature plastic parts, because the clamping plates of bar-less closing units can be better supported, thereby counteracting bending of these clamping plates and preventing formation of burrs. A further advantage of bar-less closing units can be seen in allowing free access to them for removal of the injection moulded parts and maintenance of the die plates.
For example, DE-43xe2x80x208xe2x80x2701 or EP 0 554 068 describe bar-less closing units in which the clamping plates are arranged in a C-shaped machine frame, whereby one of the clamping plates is rigidly fixed to one leg of this C-frame and the other clamping plate is displaced with the aid of a hydraulic system supported at the opposite leg of said C-frame. In order to compensate the opening of the die during the injection moulding process caused by the elastic bending of the C-frame, the injection ram is pivotably borne relative to the cylinder or the displaceable clamping plate. It is for these reasons that DE-42xe2x80x230xe2x80x2348 describes the use of a toggle lever mechanism.
In order to further reduce the bending moment caused by bar-less closing units and which occurs during the injection moulding process it has been suggested in the publications DE-U-92xe2x80x212xe2x80x2480, EP-0xe2x80x2554xe2x80x2068 or DExe2x80x243xe2x80x208xe2x80x2962 to hinge or articulate a deformable C-shaped cross-piece or frame with the stationary die carrier on the one hand, and with the hydraulic cylinder support plate on the other hand. moments which the closing force exercises upon the machine frame. However, it has been shown that even with the use of such C-cross-pieces the alignment of the plates is not guaranteed in all cases and, in particular during injection moulding processes under high pressure, the die plates do not close securely. These closing units are no longer suitable for working with highly technical plastics, where closing forces of up to 3000 N (corresponding to 300 tons) and more must be absorbed.
It is therefore the aim of the present invention to provide a bar-less closing unit for an injection moulding machine with which qualitatively impeccable (high quality) precision miniature plastic parts can be produced. In particular it is the aim of the invention to provide a bar-less closing unit having die plates which close gaplessly, even under high closing forces.
According to the present invention, this task is solved by a closing unit with the features of claim 1. In particular, this closing unit comprises rigidly mounted supporting rails having a stationary and a movable clamping plate. Behind each of these clamping plates are arranged pivot bearings in which is hinged or articulated a closing drive. This closing drive is not supported at the supporting rails nor at the machine base, but is freely movably suspended between the pivot bearings.
It has been shown that this inventive construction permits a surprisingly good die closure as compared to the above mentioned C-frame systems. This can be attributed to the fact that no torsion or bending moments are transmitted to the machine frame (machine base and/or supporting rails) during closure, and the strong closing forces also cannot cause any bending of the hydraulic piston between the displaceable clamping plate and the hydraulic cylinder support plate.
In a first preferred embodiment of the invention, two supporting rails are fixed to a machine base parallel to each other in such a way that in between them three neighbouring injection units can be arranged. These supporting rails each have an ear-like upwardly projecting support block to which the rigid clamping plate is attached. This rigid clamping plate is simply screwed to these support blocks, but it can just as well be welded thereto. It is understood that if it is appropriately dimensioned, this clamping plate can be directly attached to the support rail, thereby dispensing with the necessity of the support block. Neighbouring this rigid clamping plate is a displaceable clamping plate which is fixed to two glide shoes being movably arranged on the two supporting rails. Behind each of the clamping plates is arranged a pivot bearing, in the support block on the one hand and in the glide shoe on the other hand, and a closing drive is suspended in these pivot bearings. This means that the closing drive is freely suspended between the respective pivot bearings. Such closing drives are well known to the expert and essentially comprise two counter-moving structural components or drive components which lead to a closing or opening movement. According to the present invention, each of these two drive components of any known closing drive is connected to one of the above described pivot bearings.
In the preferred embodiment of the invention a hydraulic drive is used, whose cylinder is coupled to the pivot bearing arranged behind the rigid clamping plate, whilst the piston of this hydraulic drive is coupled with the pivot bearing arranged behind the displaceable clamping plate. This coupling is effected by lever arms which are rigidly connected to the cylinder or piston on the one hand, and pivotably engage in each of the pivot bearings on the other hand. In this way the forces of the closing unit can be directly transmitted via the U-shaped lever arms to the clamping units and thus to the die plates. In this way the die plates remain coplanar, even if increased closing forces make the reactive pressure of the die plates cause a bending of the closing drive, i.e. any possible bending of the closing drive does not result in a gap being formed between the die plates.
In a preferred further development of the inventive closing unit the respective drive parts of the closing drive are coupled with the pivot bearings over scissor-like crossed lever arms.
By freely suspending the closing drive to the pivot bearings, no bending moments are exercised to the machine base or to the supporting rails, and the die plates remain coplanar.